In the making Vesta, the second largest object in the main asteroid belt, has an iron core, a varied surface, layers of rock and possibly a magnetic field - all signs of a planet in the making, not an asteroid.

So concludes an international team of scientists treated to a virtual front row seat at Vesta for the past 10 months, courtesy of NASA's Dawn robotic probe.

They have a bit more ground to cover before Dawn leaves Vesta's cratered, lava-like surface in late August to rendezvous with the king of the asteroid belt, Ceres, another type of proto-planet believed to be flush with water ice.

Already however, the Dawn science team has confirmed long-held theories about Vesta's history, a timeline that dates back to within 300 million years of the beginning of the solar system's existence.

"We have a body that formed very, very early so we know from Vesta what conditions were like back then. We would not have learned that from a body like the Moon or Earth. We would not be able to go back that far in time," he says.

Traces of early history

From meteorites - some of which now have been directly sourced to Vesta - scientists know that the solar system is about 4.6 billion years old. Analysis of craters on the Moon date back a little more than 3 billion years. Evidence of earlier geologic lunar events were obliterated during a period of heavy bombardment, a fate Vesta apparently escaped.

Which is not to say the body, which spans about 530 kilometres in diameter, is without scars. Among the pits imaged by Dawn is a double-whammy crater - a basin inside a basin - that is believed to be the source of two groups of meteorites found on Earth.

"There is evidence in the meteoritic record to give us co-incident dates with the two impacts that we've dated on Vesta," says Russell.

Scientists also found rings around the impact basins, a phenomenon that may be related to Vesta's iron core.

"We haven't done simulations of this so we don't really know yet," says Russell. "I would guess that the iron core sort of holds Vesta in place and the (impacting body) rings around that."

Jupiter's disturbing influence

As to why Vesta never made it to full planethood, scientists point to Jupiter. When the giant gas planet formed, nearby bodies such as Vesta found their orbits perturbed.

"Jupiter started to act like a spoon in a pot, stirring up the asteroid belt and the asteroids started bumping into one another. If they're just out there gently orbiting and everything is going smoothly, then without Jupiter in the picture, they would gather mass and get bigger and bigger and bigger. But with Jupiter there, stirring the pot, then the asteroids start bumping into one another and breaking apart, so nothing grew in that region, but started to shrink," says Russell.

Vesta has been somewhat reduced by collisions, but it - unlike smaller bodies - survived.

"At the moment, we do not have any evidence of any other bodies like Vesta. It was not completely destroyed, like many other asteroids in the main belt," says planetary scientist Maria Cristina De Sanctis, with Italy's National Institute for Astrophysics in Rome.

The findings are included in six papers on Vesta published in this week's Science.